High power microwave attenuator employing a flow of lossy liquid



July 21, 1970 M. J. SHARPE 3,521,186,

HIGH POWER MICROWAVE ATTENUATOR EMPLOYING A FLOW OF LOSSY LIQUID Filed June 26, 1967 K o L E Y T s E T c R T o K n L E Y T E R AU HLM/ T 0 5 L 5- N J FIG.2 3 4,

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I v BY I mi 2900' 3100' 3500 3500' 3700 M United States Patent "ice U.S. Cl. 33381 7 Claims ABSTRACT OF THE DISCLOSURE A high power microwave attenuator is disclosed. The attenuator includes a transmission line having a pair of axially spaced quarter wave transformer radio frequency windows defining a loss section of the transmission line in the transmission line region between the windows. The loss section is filled, in use, with a lossy liquid such as water. The amount of attenuation is determined by the length of the loss section, for a given lossy liquid.

In a preferred embodiment, the windows are made of alumina or beryllia ceramic and the lossy liquid is water. The device may be employed for sampling the microwave power output of a high power device by connecting one port to the output of a high power device to be measured and connecting a microwave detector to the other port. The attenuator may also be employed as a double ended load for terminating two high power devices, such as a pair of high power klystron amplifiers.

DESCRIPTION OF THE PRIOR ART Heretofore, high power water loads have employed a quarter wave transformer window for matching an input section of waveguide to the water filled section of waveguide. An example of such a prior device is described and claimed in copending U.S. patent application 474,414 filed July 23, 1965, now U.S. Pat. No. 3,360,750 and assigned to the same assignee as the present invention.

While such prior art devices are useful as lossy terminations, they have only one microwave port and a need exists for a two port high power attenuator for attenuating power before a stage of microwave detection. Also, a two port load would be useful in permitting one load to serve two high power devices.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved high power radio frequency attenuator.

One feature of the present invention is the provision of a radio frequency attenuator including a lossy section of transmission line defined by a lossy liquid filled section of the line disposed between a pair of quarter wave radio frequency windows, whereby a two port high power broad band attenuator is obtained.

Another feature of the present invention is the same as the preceding feature wherein a microwave detector is connected to one of the ports for monitoring the output of a high power microwave source connected to the other port of the attenuator, whereby high output powers 3,521,186 Patented July 21, 1970 may be monitored without need of an expensive directional coupler.

Another feature of the present invention is the same as the first feature wherein the transmission line is an integral number of half wavelengths long between the outer faces of the axially spaced window structures when the loss section is filled with air as the dielectric fill, whereby the attenuator structures may also serve as a half wave composite output window structure for a microwave tu be.

Other features and advantages of the present invention will become apparent \upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is sectional plan view of a microwave attenuator of the present invention as connected in a circuit, shown in block diagram form,

FIG. 2 is a sectional view of the structure of FIG. 1 taken along line 2-2 in the direction of the arrows,

FIG. 3 is a graph of voltage standing wave ratio (VSWR) and insertion loss versus frequency,

FIG. 4 is a longitudinal sectional view, axially foreshortened, of a coaxial line attenuator employing features of the present invention, and

FIG. 5 is a sectional view of the structure of FIG. 4 taken along line 55 in the direction of the arrows.

Referring now to FIGS. 1 and 2, there is shown the microwave attenuator 1 of the present invention. The attenuator 1 includes a section of rectangular waveguide 2 having a pair of flanged ports 3 and 4 at its opposite ends.

A loss section of waveguide 5 is defined by a lossy liquid filled region of the waveguide defined between a pair of microwave windows 6 and 7 hermetically sealed across the waveguide 2. The loss section of waveguide 5 includes an inlet pipe 8 and an outlet pipe 9' in fluid communication therewith and through which a suitable lossy dielectric liquid such as water is passed for dissipating microwave power passed into the lossy liquid through either one or both of the microwave windows 6 and 7.

The inlet and outlet pipes 8 and 9 are directed toward the faces of the windows 6 and 7 to establish a desirable liquid flow pattern through the loss section 5, as indicated by line 11. More particularly, the cold inlet liquid is directed against input window 6 where it absorbs microwave power and is heated in the process. The heated liquid is deflected in an S-shaped course through the waveguide, across the other window 7, and out the outlet pipe 9. This S-shaped flow pattern permits both windows to be cooled and for the major cooling effect to be shifted from one window 6 to the other window 7 merely by reversing the flow of loss liquid through the pipes 8 and 9.

The microwave windows 6 and 7 form dielectric filled quarter wave transformer sections of waveguide, for matching the characteristic impedance of the air filled rectangular guide to the lossy liquid dielectric filled loss section of guide '5. It turns out that, if the lossy liquid is water and the dielectric fill of the quarter wave transformer section is alumina ceramic (dielectric constant=9), the proper waveguide height in the transformer section remains nearly the same as that in the air filled and water filled sections of guide. Therefore, one quarter guide wavelength thick slabs 6 and 7 of alumina ceramic are merely hermetically sealed across the waveguide and a relatively wide band match of 37.5% is obtained between VSWR points of 1.2 with a center passband frequency of 3300 rnHz., see FIG. 3.

In a quarter wave transformer, the quarter wave transformer section has a characteristic impedance where Z and Z are the characteristic impedances of the W veguide sections at opposite ends of the transformer section. The transformer section may be any odd integral number of quarter wavelengths long. However, the passband of the match decreases with increasing length of the transformer section. Thus, an optimum match is obtained when the transformer section is one quarter guide wavelength long at the center of the passband of interest.

Slight impedance mismatches, produced at the windows 6 and 7, are corrected by a pair of inductive irises 12 and 13 placed a quarter wavelength from the windows 6 and 7.

The attenuation (insertion loss) produced by the loss section of waveguide depends upon the length of the loss section 5 and the loss tangent of the dielectric fluid. The loss section 5 is preferably greater than one wavelength long. A typical plot of insertion loss in db versus frequency is shown in FIG. 3. Losses up to between 30 and 60 db may be achieved.

Power handling capability is limited by the waveguide power capability, the fluid flow rate of the lossy dielectric, and the specific heat of the fluid. Typically, peakpowers up to megawatts may be dissipated with average powers in the hundreds of kilowatts.

The attenuator 1 may be used to advantage in a circuit as shown in FIGS. 1 and 2 for monitoring the power output of a high power microwave source such as a klystron 15. The klystron 15 is connected to one port 3 and a microwave detector 16 is connected to the other port 4. The attenuator 1 serves as a load for the klystron as an attenuator pad between the detector 16 and the klystron 15. This arrangement avoids the requirement of a relatively expensive directional coupler which was typically used between the source and a load, in the prior art, to make such measurements.

The attenuator 1 may also serve as a load for two microwave sources by connecting a second microwave source to port 4 instead of the microwave detector 16.

Referring now to FIGS. 4 and 5 there is shown a coaxial line attenuator 21 employing features of the present invention. More particularly, the coaxial line attenuator 21 includes a cylindrical outer conductor 22 with a coaxial center conductor 23. A pair of quarter wavelength transformer radio frequency windows 24 and 25 are sealed across the transmission line 21 between the inner and outer conductors 23 and 22, respectively. The region of the coaxial transmission line between the windows 24 and 25 defines a loss section 26 of the line. A lossy dielectric fluid is piped through the loss section 26 via input and output pipes 27 and 28, respectively. The pipes 27 and 28 are tangentially directed of the loss section 26 and directed slightly in the axial direction to cause the lossy fluid to traverse a helical trajectory between input and output pipes 27 and 28.

The radio frequency windows 24 and 25 form quarter wavelength sections of solid dielectric filled transmission line. The characteristic impedances: of these quarter wave sections of the line are determined by the same relationship used for the hollow waveguide embodiments,

namely, Z Z2 where Z is the characteristic impedance of the window sections, Z is the characteristic impedance of the coaxial 4. line on the outside of the windows 24 and 25, and Z is the characteristic impedance of the lossy dielectric fluid filled section of coaxial line 26. When water is used as the lossy dielectric liquid fill of the loss section 26 and the windows 24 and 25 are made of alumina, the outer conductor 22 may have constant inside diameter throughout the attenuator 21 if the outer diameter of the inner conductor 22 is slightly recessed in the window section of the line. At 850 -mHZ., the loss section 26 is preferably about 30 inches long to provide about 20 25 db loss.

The attenuator 21 of FIGS. 4 and 5, if made an integral number of half wavelengths long between the outer faces of windows 24 and 25 assuming an air fill in the lossy section 26, will also serve as a composite halfwave window structure. In this manner, the attenuator 21 can serve, when filled with lossy dielectric fluid, as a load for a high power radio frequency tube to permit tuning and adjustment of the tube under full load conditions. After tuning and adjusting the tube, the lossy dielectric is drained from the loss section 26 and the attenuator 21 may then serve as. a half iwave output Window structure for the tube.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a radio frequency attenuator apparatus, means forming a section of transmission line having a first and second end, means forming a pair of radio frequency window structures sealed across said transmission line intermediate its ends, said radio frequency window structure being axially spaced apart along the axis of said transmission line to define a loss section of said line in the region between said spaced window structures, said loss section of the line to be filled with a lossy dielectric liquid in use for absorbing radio frequency power, each of said window structures including a block of solid dielectric material filling the window section of said transmission line, said solid dielectric blocks each having an axial length which is substantially one quarter guide wavelength long at the center frequency of the passband of the attenuator apparatus, said loss section of said transmission line having an axial length lWhlCh is longer than one guide wavelength at the center of the passband of the attenuator in the lossy dielectric liquid, whereby substantial attenuation is obtained, and wherein the portions of said transmission line which contain said dielectric blocks and which are axially coextensive therewith have a characteristic impedance with said dielectric blocks in place to form quarter wave transformer sections to substantially match the impedance of said loss section of line on the inside of said window structures to said transmission line on the outside of said window structures.

2. The apparatus of claim 1 wherein said dielectric blocks are made of alumina ceramic.

3. The apparatus of claim 1 including, means forming a microwave detector connected to said second end of said transmission line.

4. In a radio frequency attenuator apparatus, means forming a section of transmission line having a first and second end, means forming a pair of radio frequency window structures sealed across said transmission line intermediate its ends, said radio frequency window structure being axially spaced apart along the axis of said transmission line to define a loss section of said line in the region between said spaced window structures, said loss section of the line to be filled with a lossy dielectric liquid in use for absorbing radio frequency power, each of said window structures including a block of solid dielectric material filling the window section of said transloss section of line for flowing lossy dielectric fluid 5 through said loss section of line, one of said conduits being axially directed at a face of one of said window structures and the other one of said conduits being axially directed at a face of the other one of said window structures, whereby a reversal of the fluid flow through said conduits shifts the principal power absorption effect from one window structure to the other window structure.

5. The apparatus of claim 1, wherein said transmission line is a hollow waveguide structure.

6. The apparatus of claim 1 wherein said transmission line is a coaxial line.

7. The apparatus of claim 1 wherein said transmission line is an integral number of half Wavelengths long between the outer faces of said window structures when said lossy section is filled with air as the dielectric.

References Cited UNITED STATES PATENTS 3,252,034 5/1966 Priest et al. 313107 3,110,000 11/1963 Churchill 333-73 3,324,427 6/1967 Weiss 333-98 2,483,768 10/ 1949 Hershberger 33398 2,407,911 9/1946 Tonks 33398 3,360,750 12/1967 Johnson 33322 HERMAN KARL SAA'LBACH, Primary Examiner C. BARAFF, Assistant Examiner US. Cl. X.R. 

